1. Field of the Invention
The present invention relates in general to torque converters used for a transmission device for a motor vehicle and more particularly to the torque converters of a type that uses a magnetic viscous fluid instead of oil as a working fluid.
2. Description of the Related Art
Hitherto, torque converters using oil (viz., transmission oil) as a working fluid have been used in various fields for the work of torque transmission. However, in recent years, use of a torque converter using a magnetic viscous fluid having a specific weight larger than that of the transmission oil has been seriously considered for the purpose of reducing its size and improving its transmission efficiency.
Particularly, since the torque converters used in a transmission device for a motor vehicle require a lockup function, realization of a lockup control with a high accuracy by a magnetic-force control by using the magnetic viscous fluid is expected.
Such torque converter using the magnetic viscous fluid is disclosed in Japanese Laid-open Utility Model Application (Jikkaihei) 07-002663.
In this known torque converter using the magnetic viscous fluid, an exciting coil is arranged to extend around the vicinity of an outer peripheral edge where a pump impeller and a turbine runner are arranged to face each other, an induction coil and an electromotive coil are sequentially provided on the outside of the exciting coil, and electric power is supplied from the electromotive coil to the exciting coil via the induction coil.
When the magnetic viscous fluid is magnetized by a magnetic force by the exciting coil, the yield stress (viz., apparent viscosity) of the magnetic viscous fluid is changed. Thus, in the known torque converter, a slip resistance between the pump impeller and the turbine runner is varied by changing the magnetic force, and when rotation on a loaded side has not sufficiently risen, the turbine runner is forced to generate a torque by using an inertia force of the magnetic viscous fluid without making the magnetic force control to the magnetic viscous fluid, and when the rotation on the loaded side has risen, the magnetic force is applied to the magnetic viscous fluid so as to reduce the slip between the pump impeller and the turbine runner.